Vampyroteuthis

I was kinda just wondering the same thing. But Im more worried about them being eaten than releasing toxics into the water. Which no they dont unless attacked, which only some species do. But bear in mind they are VERY fragile. But other than that I think that it is a great idea. I would like to keep a spanish dancer with an octo. I have been hunting for one for a long time now but I have the worst of luck when it comes to that sort of thing. :P Oh yeah by the way I was keeping a sea hare (which is a ophistobranch species of nudibanch) with my octo before my tank crashed and they really never bothered each other. Good luck on keeping a nudibanch they are soooo cool. :D

Vampyroteuthis

Yeah that is kinda true but I have found that most ophistobranchs do really well in captivity and some have been kept for years at a time! Not to mention they are the most adverse kind of nudibanch so there is lots of different shapes sizes and colors to choose from. :)

Colossal Squid

Personally i have never had much luck on keeping nudibranchs at all, let alone with a cuttle...

Perhaps the cuttle would recognise the nudi as being toxic or maybe not??? Hard one to guess and its another one of those suck it and see situations... dont think i'd try it until you have a bit more experience with just the cuttles...

Sepia elegans

Spanish dancers average size is 30cm, I think the largest was about 45cm...
As for the swimming, you sure? My old Smithsonian MarineBio book says it will swim or crawl, depends on individual, but it is from 1996, so... neat if I'm wrong, but it'll still need a large tank...